Effect of segmented telescope phasing errors on adaptive optics performance Marcos van Dam Flat Wavefronts Sam Ragland & Peter Wizinowich W.M. Keck Observatory
Motivation Keck II AO / NIRC2 K-band Strehl ratios from 2007 Image quality limited by error terms independent of seeing!
Motivation Keck II AO / NIRC2 K-band Strehl ratios from 2007 Strehl = 75% Best images have low order static aberrations!
Outline of Talk Review of Keck telescopes, phasing and AO systems
Outline of Talk Review of Keck telescopes, phasing and AO systems Measuring phase discontinuities with a Shack-Hartmann WFS
Outline of Talk Review of Keck telescopes, phasing and AO systems Measuring phase discontinuities with a Shack-Hartmann WFS Results of on-sky phase retrieval experiments
Keck Telescopes Two twin 10-m telescopes on Mauna Kea 36 hexagonal segments NGS/LGS AO on both telescopes
Phasing Camera Optically measures phase between adjacent segments 78 measurements used to contrain 36 segment pistons Phase is maintained with capacitive edge sensors aided by look-up tables Temporal stability of phasing not well understood
Phasing Errors Random errors in phase measurements lead to low spatial frequency segment piston errors Eigenmodes
AO systems Both telescopes have almost indentical AO systems 20x20 Shack-Hartmann WFS with quad cells NGS and LGS 21x21 actuator Xinetics DM (349 actuators)
Response of SH WFS to phase discontinuities Centroid changes in response to phase discontinuity
Response of SH WFS to phase discontinuities Centroid is exactly the same for a discontinuity as for a constant slope
Response of SH WFS to phase discontinuities Quad cell is even more sensitive to phase discontinuities
Response of SH WFS to phase discontinuities End-to-end simulations were run in yao to see effect of phasing errors on image quality Applied phasing errors
Response of SH WFS to phase discontinuities End-to-end simulations were run in yao to see effect of phasing errors on image quality Marechal approximation Phasing error partially corrected
Response of SH WFS to phase discontinuities End-to-end simulations were run in yao to see effect of phasing errors on image quality Applied telescope phase Residual error Take home message: Small phasing errors are measured and partially corrected, large errors are not!
Phase retrieval from images 50 short-exposure images were taken in focus Modified Gerchberg-Saxton algorithm used to reconstruct phase
Phase retrieval from images
Phase retrieval from images 50 short-exposure images were taken in focus Modified Gerchberg-Saxton algorithm used to reconstruct phase Average the reconstructed phases, but there is a phase ambiguity due to the pupil symmetry
Phase retrieval from defocused images 50 short-exposure images were taken either side of focus Used two different methods to reconstruct: Modified Gerchberg-Saxton algorithm Non-linear minimization in yorick-opra software Results are almost identical, with RMS value of 112 nm In-focus images Modified Gerchberg-Saxton Defocused images Modified Gerchberg-Saxton Defocused images yorick-opra
Conclusions Performance of Keck AO systems on bright stars is limited by phasing errors Keck II AO / NIRC2 K-band Strehl ratios from 2007
Conclusions Performance of Keck AO systems on bright stars is limited by phasing errors Random phase errors in segmented telescope lead to low-order phase errors
Conclusions Performance of Keck AO systems on bright stars is limited by phasing errors Random phase errors in segmented telescope lead to low-order phase errors Shack-Hartmann WFS can measure ~100 nm RMS segment piston
Conclusions Performance of Keck AO systems on bright stars is limited by phasing errors Random phase errors in segmented telescope lead to low-order phase errors Shack-Hartmann WFS can measure ~100 nm RMS segment piston and DM can partially correct it
Conclusions Performance of Keck AO systems on bright stars is limited by phasing errors Random phase errors in segmented telescope lead to low-order phase errors Shack-Hartmann WFS can measure ~100 nm RMS segment piston and DM can partially correct it Phasing errors can be estimated from AO-corrected images at or near focal plane
Future work Test a method called phase discontinuity sensing that uses very defocused images. Use the output of the phase reconstruction to update the telescope segment phasing